Atmospheric pressure chemical vapor deposition (APCVD) is a semiconductor processing technique that is well known in the art. Several companies make APCVD apparatus for sale to semiconductor manufacturing companies. The present invention is an improvement to a prior art APCVD arrangement, as is described in detail in this specification.
U.S. Pat. No. 4,834,020 (Bartholomew, et al.) discloses a conveyorized atmospheric pressure chemical vapor deposition apparatus. This patent is assigned to Watkins-Johnson Company of Palo Alto, Calif. The apparatus described in this patent discloses use of nitrogen blanketing apparatus surrounding each deposition chamber to separate the ambient atmosphere from the deposition environment and to ensure that deposition chemicals do not escape from the immediate vicinity of the deposition chamber. Nitrogen distribution plenums are positioned on the entry side of each deposition chamber and on the exit side of each deposition chamber.
U.S. Pat. No. 5,304,398 (Krusell, et al.) also is assigned to Watkins-Johnson Company. This patent also discloses an atmospheric pressure chemical vapor deposition apparatus, which includes a gas injection assembly (indicated at reference 310 in FIG. 1 of the present specification, which is a reproduction of FIG. 3 of the Krusell, et al. patent) through which process gases and nitrogen are introduced onto the surface of wafer 315 that is processed by the apparatus. This is the type of system in which the preferred embodiment of the present invention is intended to be used. Referring to FIG. 1, the structures 12, 14a and 14b on either side of exhaust passages 336 and 338 are nitrogen distributors which provide a nitrogen blanket around the process area and the gas injection assembly 310. The function of these nitrogen distributors is to provide a steady flow of nitrogen to the regions surrounding the process gas injection operation to remove excess or spent process gasses and to prevent introduction of foreign materials into the process area. The nitrogen that is fed through these distributors is removed from the process area via exhaust passages 336 and 338, thus forming a continuous flow of inert gas surrounding the process area. The aforementioned patents are hereby incorporated by reference into this specification.
FIG. 2A shows the construction of a prior art nitrogen distributor assembly over which the present invention provides a number of improvements. Nitrogen distributor assembly 10 comprises primary nitrogen distributor 12 and secondary nitrogen distributors 14. Each distributor 12, 14 comprises a solid shell 20 and a perforated screen 24 that form an enclosed space into which nitrogen may be introduced. Nitrogen supply tubes 16 are provided to convey nitrogen from an external nitrogen source to the nitrogen distributor assembly. Inside of nitrogen distributors 12 and 14, these tubes 16 are perforated to release nitrogen within the distributors. Referring to the prior art primary distributor 12, each nitrogen tube 16 enters the distributor and is bent at a 90.degree. angle as shown at point 18 and then welded at selected points along its length to shell 20 of primary distributor 12 beneath screen 24. A number of holes are drilled in nitrogen tube 16 proximate shell 20 in order to allow nitrogen to flow from the tube 16 into the enclosed space formed between shell 20 and screen 24 of primary nitrogen distributor 12. Baffle 22 is constructed from a thin strip of metal and attached to tube 16 at one edge and to shell 20 at the other edge. Baffle 22 is provided in order to block the direct flow of nitrogen out of the perforations in tube 16, to slow down the nitrogen flow and to allow it to evenly disperse within primary distributor 20 beneath primary screen 24. A similar structure is employed in forming secondary nitrogen distributors 14 by using a corresponding tube having holes drilled therein positioned between a secondary shell a secondary stainless steel screen.
The prior art nitrogen distributor assembly shown in FIGS. 2A and 2B employs topographic features 26 in the primary screen 24 that are intended to control temperature-cycling related failures. Because these features exist only in the planar section of the primary screen 24 and do not extend around the curve that is formed by the screen as it passes around the lateral sides of primary distributor 12, they tend to ensure that thermal-cycling failures occur primarily in the specific region of the features, rather than preventing such failures.
FIG. 2B shows a typical prior art version of a APCVD injection assembly including gas blanket distribution apparatus. A work piece, which is typically a semiconductor wafer 210, may be conveyed past the operative assembly on conveyor belt 212. Injector head 214 receives the reactant process gasses and furnishes them through injector nozzle 216, which extends through a window formed in primary nitrogen distributor 218, and onto the surface of work piece 210 as it passes beneath nozzle 216. Secondary nitrogen distributors 220 are provided on each side of primary nitrogen distributor 218. The sides of primary nitrogen distributor 218 and secondary nitrogen distributors 220 that face each other and that face toward the work piece comprise perforated screen that may be made of stainless steel, which allow nitrogen to flow from the interiors of primary distributor 218 and secondary distributors 220 and into the vicinity of work piece 210 and injector nozzle 216 such that excess and spent process gasses are diluted and carried by the nitrogen flow away from the process area and up through the exhaust channels provided between primary nitrogen distributor 218 and secondary nitrogen distributors 220. The flow of nitrogen is generally shown by the arrows in FIG. 2B.
Nitrogen supply tubes 222 are operatively connected to a source of nitrogen (or other selected blanket gas) to provide a steady flow of nitrogen into primary nitrogen distributor 218 and secondary nitrogen distributors 220. Additional tubes 224 may be used to provide reactant gasses into injector head 214. First, the secondary distributor seals used in prior art devices are subject to leaking and failure. This is due in part to the design of prior art sealing elements and in part to inadequate structural rigidity in the distributor assembly. Second, prior art devices are prone to cause "powdering," which can introduce harmful particulate contaminants into the processing area. Third, the prior art nitrogen distributor assemblies are difficult to manufacture. Finally, the perforated screens in prior art devices are known to experience stress cracking due to differential rates of thermal expansion between distributor components. Each of these deficiencies is discussed further below.
In the atmospheric pressure chemical vapor deposition (APCVD) machines that are known to the applicant, the nitrogen blanketing apparatus that is employed has several deficiencies which are corrected by the present invention.
Secondary seals 226 may be provided on the outboard side of each secondary distributor 220. Each secondary seal 226 provides a seal between secondary nitrogen distributor 220 and the wall of the enclosure into which this assembly is placed during operation, which is not shown in this illustration. (See FIG. 7) In the prior art, as shown in FIG. 2B, secondary seal 226 is a strip of flexible stainless steel, one edge of which is welded to the back of secondary nitrogen distributor 220, and the other edge of which is bent away from secondary distributor 220 in order to contact the enclosure when the distributor assembly is inserted therein. One difficulty with this arrangement is that in the event that secondary distributor 220 bends or flexes during operation due to mechanical or thermal stresses, secondary seal 226 tends to buckle thereby lose its contact with the enclosure wall over a portion of its length, and a leak in the seal can therefore be formed. One deficiency in the prior art is that the secondary distributors are not adequately stiff to resist bending as a result of mechanical and thermal stresses applied during operation, and when such bending occurs the secondary seals can become incapable of closing the space between the back of the secondary distributor and the enclosure wall.
Furthermore, prior art seals have a spring rate that is relatively large compared to the stiffness of the secondary assemblies. Thus, when the distributor assembly is forced into its enclosure, a force is developed that distorts the straightness of the secondary distributors , non-linearly distorting the gap between primary and secondary distributors. This distortion in the gap width affects the uniformity of the process by introducing unpredictable variations in the shielding gas flow pattern. Another problem is that the prior art seals are non-conformal against irregularities in the walls of the enclosure they fit within. Some prior art seals consist of a thin strip of metal incorporating a single bend away from the body of the secondary shell. A serious problem with this design becomes apparent when installation into the enclosure is attempted. Because the strip opens outward into the enclosure, there is a tendency for the leading edge of this seal to be captured by irregularities in the sealing surface causing the thin metal to buckle or suffer an otherwise catastrophic failure. An alternative prior art seal design incorporates a reentrant bend along the distal edge of the seal. This bend makes the distributor easier to insert into its enclosure, but because of the reentrant bend, the sealing edge is more rigid and therefore even less conformal than the aforementioned design.
A phenomenon referred to as "powdering" is known to occur in prior art nitrogen blanket injection assemblies. This occurs when the flow of nitrogen and process gasses becomes restricted and silicon or some other solid material becomes deposited upon surfaces within that apparatus. Those deposits can break loose and form particulate impurities which can adversely affect the semiconductor manufacturing process. In addition to providing a shielding atmosphere for the process, the nitrogen passing through the perforations in the screens serves to prevent such reaction products from building up on the outside of the distributor assemblies. Some prior art nitrogen distributors have areas of the screen that are blocked so that nitrogen cannot flow therethrough. For example, some designs include an overlying piece of the screen material around each tube penetration that in effect blocks many of the perforations in those areas. A similar problem is caused by a feature sometimes added in an attempt to stiffen the secondary assembly, wherein the edge of the secondary shell secondary is bent back upon itself. This forms a solid surface that lies beneath the perforated screen material and eliminates any possible gas flow. These features are believed to be largely responsible for producing the powdering effect noted in the prior art.
Another deficiency in the prior art nitrogen distribution assemblies is that they are not conducive to mass production manufacturing techniques, primarily because the nitrogen supply tubes are an integral part of the plenum assembly. Once they are welded to the primary and secondary shells early in the manufacturing process, the supply tubes become cumbersome impediments during the remaining stages of production. It would be preferable to be able to install the nitrogen supply tubes as a final step in the manufacturing process.
Yet another deficiency in the prior art is that the secondary distributors are relatively long, thin assemblies which may be prone to bending or flexing during use. This is undesirable because it is important to provide a reliable seal between the outboard side of the secondary distributors and the enclosure into which the assemblies are installed. It has been noted that the prior art secondary distributors are not stiff enough to prevent flexing due to mechanical forces, primarily the forces exerted by prior art seal structures. Furthermore, the seals that are used on the outboard sides of the prior art secondary distributors are not adequate, when combined with the flexibility of the secondary distributors to provide a reliable seal during use of the apparatus.
Both the primary nitrogen distributor and the secondary nitrogen distributors in the prior art have outer surfaces made of stainless steel screen material. In use, nitrogen flows from the nitrogen distribution apparatus within the nitrogen distributors and outward through the screen material into the regions surrounding the wafers being processed. During operation of these machines, the temperature of the processing chamber can fluctuate between room temperature and 600.degree.. This temperature cycling results in flexing or "oil canning" of the perforated screen material, due to differences in the rate of thermal expansion between the screen and the shell to which it is attached. Repeated stressing of the screen material in this way results in a common mode of failure where cracks form in the surface of the screen. These cracks interrupt the desired nitrogen flow patterns and can cause undesired turbulence in the vicinity of the cracks, decreasing the effectiveness of the nitrogen blanket and the APCVD process. Some prior art distributors have features 26 formed in the screens that serve to localize stress, but the known approaches for providing stress relief are insufficient to cure the "oil canning" problem.
It is therefore desirable to provide a nitrogen blanket distribution apparatus for an atmospheric pressure chemical vapor deposition process for semiconductor processing which reduces powdering, improves manufacturability, provides stress relief for the screen material, provides improved stiffness of secondary nitrogen blanket distributors, and provides improved sealing between the secondary distributors and the walls of the enclosure in which the nitrogen blanket distributor assembly is located.